(1) Field of the Invention
The present invention relates to a method and system for the production of coated fibers. The present invention particularly relates to a composite prepreg with either a thermoplastic or thermoset matrix on fibers in a tow which is suitable for use in primary and secondary structural preform methods with significant cost, speed and environmental advantages. The present invention particularly relates to a method and system which uses a vibrating means in a housing to fluidize particles and to coat them on the fibers.
(2) Prior Art
High-speed, low-cost processing methods are required to produce prepreg composite materials for high volume applications with either thermoset or thermoplastic matrices dispersed through a tow of fibers. Conventional processes like hot melt prepregging rely on the creation of a low viscosity matrix phase early in the process which is then forced around and through the fiber tows at high pressure. Wetting of individual fibers at the microscopic level is the ultimate goal. For thermoset materials, dilution in solvents has been relied upon for composite fabrication or if the reaction window is long enough, elevated temperatures are used to lower resin viscosity. Thermoplastic matrices are inherently high in viscosity and elevated temperature and pressure can not always be used. Solution methods are commonly used. Environmental and health concerns as well as the costs associated with using and reclaiming high boiling solvents makes their use with these methods undesirable. Many high performance matrix resins are not soluble in any solvents and they are therefore not capable of being used in high volume applications at the present time.
The key feature to any successful processing method is the ability to place the polymer around the fibers to form the prepreg. A viable processing method should place the polymer in its final position on the fiber surface and should be applicable to any matrix.
(1) Solution Processing:
This method is used with both thermoset and thermoplastic matrices. Polymer is dissolved in a solvent and the fiber tow is impregnated with the resulting low viscosity solution (Turton, N., and J. McAinsh, U.S. Pat. No. 3,785,916). Complete removal of solvent after impregnation is a stringent requirement and is often a difficult step. Methylene chloride, acetone and N-methyl pyrrolidone are widely used as solvents. Epoxies, polyimides, polysulfone, polyphenyl sulfone and polyether sulfone are some of the matrices which have been solution-impregnated.
(2) Slurry Processing:
Polymer particles are suspended in a liquid carrier and the fiber tow is passed through a slurry tank which contains polymer particles. Polymer particles are trapped within the fiber tow. Taylor (Taylor, G. J., U.S. Pat. No. 429,105) outlines a process whereby the particles are suspended in water which is thickened with a material such as polyethylene oxide to increase the viscosity to 400-3000 cP at 25 C. O'Connor (O'Connor, J. E., U.S. Pat. No. 4,680,224), points out problems with this method which include finding the right concentration of slurry, maintaining the optimum concentration in the resin tank and accumulation of excess resin at the die entrance (where the impregnated tow is consolidated into a tape or flat sheet). The minimum void contents in the processed tape were about 2 to 4%. Another method (Dyksterhouse, R., Dyksterhouse, J. A., U.S. Pat. No. 4,894,105) impregnates the fibers in a gelled impregnation bath with plastic flow characteristics, shear-thinning behavior and a polymer binder in which the polymer particles are uniformly suspended. A PEEK-carbon fiber void-free composite was made by impregnating G30-500 carbon fibers in a gelled impregnation bath having a viscosity of 81,000 cP.
(3) Melt Impregnation:
Direct impregnation of the fiber tow with molten polymer is possible. For thermoset matrices like epoxy, temperature and reaction kinetics allow for a continuous melt impregnation before reaction. For thermoplastics, two approaches have been used: (a) a cross head extruder feeds molten polymer into a die through which the rovings pass (Moyer, R. L., U.S. Pat. No. 3,993,726), and, (b) the fibers pass through a molten resin bath fitted with impregnation pins to increase the permeability of the polymer into the tow. The impregnation pins can be heated to decrease viscosity locally to further improve the impregnation process (Cogswell, F. N., et al. U.S. Pat. No. 4,559,262). In either case, the force exerted on the fibers e.g. die pressure for the crosshead extruder are extremely high and can cause fiber damage. The resulting prepreg usually lacks tack and drape.
(4) Film Stacking:
Layers of fiber reinforcement either in the form of unidirectional tows or woven fabrics are stacked with thermoplastic sheets as the matrix material and consolidated under pressure for long times (Lind, D. J., and Coffey, V. J., U.K. Patent 1,485,586). This method is widely used due to the relative ease of manufacture. Disadvantages include high resin content, the uneconomic (labor-intensive) nature of the process and difficulty in impregnating the fiber tow (high pressure forces the fibers together) with the high viscosity matrix material.
(5) Fiber Co-mingling:
A thermoplastic matrix can be spun into a fine yarn and co-mingled with the reinforcing fiber tow to produce a co-mingled hybrid yarn (Clemans, S. R., et al., Materials Engineering, 105, 27-30 (March 1988)). These hybrid yarns can be consolidated to form composite parts. An advantage of this technique is the drapeability of the hybrid yarn. The high cost involved in producing thermoplastic yarn and weaving it with the reinforcing fibers is a disadvantage.
(6) Dry Powder Impregnation:
Dry thermoplastic powder is introduced into a fiber tow which is then processed by heating to sinter the powder particles onto the fibers. This technique was first employed by Price (Price, R. V., U.S. Pat. No. 3,742,106) who passed glass roving through a bed (either fluidized or loosely packed) of thermoplastic powder. Polypropylene particles with an average diameter of 250 microns were used. The particles stick to the fibers due to electrostatic attraction. The tow is then heated and passed through a die to produce an impregnated tow. The impregnation is macroscopic, i.e. the particles coat clusters of fibers rather than individual fibers leaving unwetted areas and voids. The process is targetted mainly at producing short fiber reinforced thermoplastics. Ganga (Ganga, R. A., "Flexible Composite Material and Process and Apparatus for Producing Same", AT0113 (DPI8176); "Procede de Fabrication d'Objets Composites Obtenus", FR 2548084-Al, June 1983), fluidized polyamide particles less than 20 microns in size in a fluidization chamber, impregnated glass rovings and covered this with an outer sheath of a second material of lower melting point than the impregnated particles. The second sheath was extruded onto the tow. Muzzy (Muzzy, J. D., ASME Symposium on Manufacturing Science of Composites, p27-39 (April 1988)) recently demonstrated the ability to manufacture prepreg by passing a spread tow through an electrostatic fluidized bed of PEEK powder (50 microns).
Dry powder processes offer the optimum potential for the creation of a high speed, low-cost process if the creation of a void-free composite with controllable volume fractions can be achieved. An ideal process should have the following characteristics:
(i) It should be independent of matrix viscosity. Most high performance thermoset and thermoplastic matrices are either very reactive at the high temperatures necessary to reduce their viscosity or are highly viscous (10.sup.3 to 10.sup.5 Poise) above their softening point (amorphous) or melting temperature (semi-crystalline). A viable dry powder process circumvents this problem by coating fibers individually with the required amount of matrix so that flow of the polymer takes place only over submicron distances between particles rather than centimeter distances from the outside to the inside of the fiber tows.
(ii) It should avoid the use of binders or solvents which have to be evaporated during the latter stages of the processing cycle. This can always be a source of voids which have a deleterious effect on the mechanical properties of the composite.
(iii) The average particle size of the material used is preferably approximately the same as the dimensions of the fiber for optimum impregnation. A significantly larger particle size will cause bridging and restrict impregnation due to physical limitations which results in a non-uniform distribution of resin between the fibers.
(iv) The concentration of powder particles in the "impregnation chamber" where they meet the fibers should be constant and controllable at all times.
(v) The mechanism of adhering the particles to the fibers should be controllable and independent of environmental conditions.
(vi) The resulting prepreg tape should be in a form that is flexible and drapeable so that complex parts can be formed easily.
(vii) The process should require minimal energy use, be free of labor intensive steps, capable of operating at high speeds and be capable of scale-up to large sizes.
Patents which relate to fiber handling which are of general interest are U.S. Pat. Nos. 2,244,203 to Kern; 3,017,309 to Crawford; 3,304,593 to Burklund; 4,534,919 to McAliley et al; and 4,714,642 to McAliley et al.